Ethylene polymers derived from at least one polar monomer with one or more ester groups, which polymerize with ethylene (e.g., polymers derived from vinyl acetate or other vinyl esters of monocarboxylic acids such as poly(ethylene-co-vinyl acetate) (EVA) or copolymers derived from ethylene and an acrylate ester or methacrylate ester such as ethylene n-butyl acrylate (EnBA)), are used in a wide range of applications. For example, EVA is commonly employed in hot-melt adhesives for use in papers and packaging, in conjunction with non-woven materials, in adhesive tapes, in electrical and electronic bonding, in general wood assembly, in road marking and pavement marking applications, and in other industrial assembly. EnBA is used in various hot-melt adhesive applications, including low application temperature hot-melt adhesives. EnBA has a relatively low glass transition temperature Tg as compared to EVA. EnBA based hot-melt adhesives can offer higher adhesion even to difficult substrates, better thermal resistance, increased adhesion to metals and glass, and can offer beneficial low temperature use properties. Hot-melt adhesive comprising a mixture of relatively low molecular weight ethylene copolymers, have also found utility as hot-melt adhesives that can be applied at temperatures of from about 100° C. to about 150° C. which have good bond strength with exceptional toughness, good heat resistance and acceptable application viscosity. Examples are relatively low molecular weight EnBA copolymers having a high melt flow index value, or blends derived from an EnBA copolymer such as a blend with an EVA copolymer, that can lead to hot-melt adhesives which can be applied at temperatures of from about 100° C. to about 150° C., having good bond strength with exceptional toughness, good heat resistance and acceptable application viscosity.
Many commercially available hot-melt adhesives require temperatures of 177° C. or greater to ensure complete melting of all the components and also to achieve a satisfactory application viscosity. The high temperatures increase safety risks such as burns and residual volatiles inhalation. In addition, the use of high temperatures requires more energy. Adhesive formulations have been developed that can be applied at temperatures below 150° C., preferably below about 140° C., more preferably below about 135° C., even more preferably at about 120° C. down to about 100° C. Typically, low molecular weight polymers or copolymers having a relatively high melt flow index, also often referred to as melt index (MI), e.g. having MI values of e.g. 400 grams/10 minutes or higher, or alternatively having MI values of 750 grams/10 minutes or higher, are applied therein. Examples of applied polymers in such low temperature application hot-melt adhesives are EnBA copolymers, or EVA copolymers, having MI values of e.g. 400 grams/10 minutes or higher, or having MI values of 750 grams/10 minutes or higher, or blends thereof. While not bound by any particular theory, it is believed that in order to prevent a loss of adhesive properties such as toughness, heat resistance and specific adhesion to a substrate in such low temperature application hot-melt adhesives, a tackifier or resin such as a rosin ester with a relatively high average molecular weight and relatively high softening point can be applied to render acceptable adhesive properties, such as heat stress resistance (HSR). Tackifiers or resins such as rosin esters with increased average molecular weight values (Mn, Mw and in particular higher power average molecular weights such as Mz and Mz+1 expressed as gram/mol) offer an advantage by contributing to better adhesive properties, such as heat resistance performance of the low application temperature hot-melt adhesives derived from them. Low temperature application hot-melt adhesives in general will also contain a wax such as a low melting Fischer-Tropsch wax or a paraffin wax.
It can be deduced e.g. by applying statistical mechanics that in general the mixing of higher molecular weight components will more likely lead to a less favorable free energy of mixing (ΔGm) which can be attributed to the resulting less favorable entropy of mixing contribution (ΔSm). The free energy of mixing is related to the entropy of mixing: ΔGm=ΔHm−T·ΔSm, wherein ΔHm represents the enthalpy of mixing and T represents the absolute temperature. The ΔSm term will be greater than zero upon mixing molecules from different components such as rosin ester molecules and polymer molecules but the value of ΔSm will generally decrease with increasing molecular weight values of the mixed components, i.e., the T·ΔSm term in the thermodynamic ΔGm=ΔHm−T·ΔSm equation will in such a case become relatively smaller.
This relatively smaller entropy of mixing (ΔSm) contribution to the free energy of mixing (ΔGm) in the case of mixing larger molecular entities will generally result in a worsened degree of compatibility of the components in the resulting mixture. The detrimental effect of increasing resin molecular weight above a critical Mw value on resin-polymer compatibility has been reported e.g. in J. B. Class and S. G. Chu. The viscoelastic properties of rubber-resin blends. II. The effect of resin molecular weight. Journal of Applied Polymer Science 1985, 30, 815-824, which is incorporated herein by reference in its entirety.
Ethylene copolymers, as exemplified by ethylene and vinylalkanoate monomer based polymers, e.g. EVA, or ethylene and acrylate or methacrylate monomer based polymers, e.g. EnBA, in general do not contain aromatic rings or aromatic moieties in their chemical structure. A popular aphorism, well known to a person skilled in the art, which relates to the solubility or compatibility performance in mixing different chemical components, is ‘like dissolves like’. Essentially, this ‘like dissolves like’ expression is related to the enthalpy of mixing (ΔHm) contribution in the thermodynamics equation of the free energy of mixing (ΔGm): ΔGm=ΔHm−T·ΔSm. Therefore, it can be rationalized that the degree of aromaticity of a rosin ester preferably should be as low as possible in order to contribute to a low positive enthalpy of mixing (ΔHm) value, or even more preferably to a negative value of ΔHm in the hot-melt adhesive formulating process, when being mixed with a polymer having a low aromatic content or with a non-aromatic polymer component. A relatively low degree of rosin ester aromaticity is thereby anticipated to contribute to a more optimal, i.e. negative, free energy of mixing (ΔGm) value and thereby can exert a positive impact on the resulting degree of hot-melt adhesive compatibility. Aromaticity is defined as the relative number of aromatic carbon atoms and aromatic hydrogen atoms attached to an aromatic ring. Aromaticity can be analyzed via nuclear magnetic resonance (NMR) spectroscopy, e.g. by determining the relative number of hydrogen atoms attached to an aromatic ring, or via cloud point determination using an appropriately chosen solvent system. More information on the effect of tackifier aromaticity on adhesive performance can be found in O'Brien, E. P.; Germinario, L. T.; Robe, G. R.; Williams, T.; Atkins, D. G.; Moroney, D. A.; Peters, M. A. Fundamentals of hot-melt pressure-sensitive adhesive tapes: the effect of tackifier aromaticity. J. Adhesion Sci. Technol. 2007, 21, 637-661, which is incorporated herein by reference in its entirety.
These enthalpic and entropic contributions to the free energy of mixing especially can become a critical technical issue in relation to low application temperature hot-melt adhesive compatibility since the resin molecular weights that are applied in such low application temperature hot-melt adhesives in general will have to be increased to a higher level in order to adjust the hot-melt adhesives HSR performance to a higher level which is needed for industrially applicable low application temperature hot-melt adhesives. In such industrially applicable low application temperature hot-melt adhesives, the resulting compatibility can be expected to easily approach or exceed the limit of incompatibility. Besides the impact of the molecular weight distribution of the resin, the softening point of the resin as well as hot-melt adhesive viscosity can be considered as relevant factors with regard to adhesive heat stress resistance performance. The underlying structure-property relationships are complex and not fully understood as is for example outlined in a publication authored by Ambrosini, Heat stress resistance of hot-melt adhesives, pp. 166-170, September 1993 Tappi Journal, which is incorporated herein by reference in its entirety.
While not bound by any particular theory, it is believed that an increase in hot-melt adhesive viscosity will generally be associated with a gradual increase in the average molecular weight of the chemical components in the hot-melt adhesive.
In cases wherein compatibility is already near the critical threshold like will be the case in many low application temperature hot-melt adhesive applications it can be expected that such an increase in molecular weight of the chemical components in the hot-melt adhesive can aggravate incompatibility and thereby lead to a lower degree of adhesive performance of the hot-melt adhesive.
It can be important that the degree of viscosity stability of hot-melt adhesives within the applied application temperature range of 100° C. and higher, is high. It can be expected that a high degree of thermal viscosity stability will contribute to improved hot-melt adhesive compatibility and thereby can prevent a detrimental adhesive performance of the hot-melt adhesive.
In many cases, for example when used in hot-melt adhesive formulations, ethylene polymers derived from at least one polar monomer with one or more ester groups, which polymerize with ethylene are processed at elevated temperatures of 100° C. or higher. In these applications, it is important that the polymers exhibit viscosity stability at elevated processing temperatures and remain stable and compatible in the hot-melt tank during processing and in between different processing runs. For example, in the case of hot-melt adhesive formulations, changes in the viscosity of the adhesive upon incubation at an elevated processing temperature jeopardize the quality of an adhesive bond or joint formed using the hot-melt adhesive. In addition, an increase in hot-melt adhesive viscosity can be associated with an increase in molecular weight and with gelling. Gelling in the hot-melt adhesive formulation can negatively impact hot-melt adhesive clean running properties. This can lead to hot-melt equipment nozzle obstruction or can aggravate such an obstruction which can increase hot-melt equipment downtime. Unfortunately, ethylene copolymers which are copolymers with one or more polar monomers which contain an ester group and which polar monomers can polymerize with ethylene, such as EVA or EnBA, can exhibit limited viscosity stability at elevated temperatures such as in the case of hot-melt adhesive formulations. While not bound by any particular theory, it is known that ethylene polymers derived from at least one polar monomer with one or more ester groups, which polymerize with ethylene can be thermally unstable such as in the case of hot-melt adhesive formulations. At elevated temperatures, they can degrade, which can lead to crosslinking of the copolymer and an increase in viscosity. By stabilizing the viscosity of ethylene polymers derived from at least one polar monomer with one or more ester groups, which polymerize with ethylene (e.g., copolymers derived from ethylene and vinyl acetate or n-butyl acrylate) at elevated temperatures, but also at relatively lower application temperatures such as in the range of 100° C. to 150° C., in hot-melt adhesive formulations, the processing of such materials can be greatly improved.
Low application temperature hot-melt adhesives based on maleic anhydride fortified rosin esters are known (EP 1,522,566 A2 to Haner, which is incorporated herein by reference in its entirety). The production of such Diels-Alder or Ene reaction fortified rosin esters requires an additional reaction step, viz. the reaction of a dienophile such as maleic acid (or maleic anhydride) or fumaric acid with rosin prior to the esterification reaction in order to increase molecular weight and carboxyl group functionality. Therefore, the application of an additional Diels-Alder or Ene reaction step on top of rosin esterification can add additional complexity and cycle time to the rosin ester production process as compared to a plain rosin ester production process which is primarily based on esterification. The rosin esters in the present invention lack such a Diels-Alder or Ene reaction step and are based on esterification as the main chemical reaction type to achieve the required rosin ester molecular weight distribution and softening point. Rosin esters based on a rosin, a polyol and aromatic dicarboxylic acids, so-called aromatic dibasic acids, such as isophthalic acid or terephthalic acid can also be used in hot-melt adhesive applications. Such rosin esters are described in U.S. Pat. No. 5,120,781 to Johnson, which is incorporated herein by reference in its entirety. Rosin esters based on rosin, a polyol and aromatic dicarboxylic acids such as isophthalic acid or phthalic acid will have a higher relative aromatic content than corresponding rosin esters devoid of such an incorporated aromatic moiety. The resulting higher relative aromatic content can go at the expense of ethylene-vinyl acetate (EVA) or ethylene-n-butyl acrylate copolymer (EnBA) copolymer compatibility as these two copolymer structures lack aromaticity. As a result, the heat stress performance contribution of such rosin esters, as exemplified by SYLVALITE™ RE 110L and SYLVALITE™ RE 105L, in a low temperature hot-melt adhesive application, which have to remain EVA and EnBA compatible, can become insufficient for demanding industrial adhesive applications. Rosin esters, to be used in hot-melt adhesives, can have food contact approval. The monomeric components that can be used in the production of such food contact approved rosin esters preferably are listed on the EU Plastics Regulation (Regulation (EU) No. 10/2011 on plastic materials and articles intended to come into contact with food).
Based on considerations as outlined above there exists still a clear commercial and industrial need for rosin ester resins to be applied in low temperature application hot-melt adhesives which rosin esters positively contribute to important hot-melt adhesive properties in terms of performance (for example heat stress resistance, adhesive (co)polymer compatibility, thermal oxidative-, color-, and viscosity stability, shelf life and adhesion) and which on top of that align to regulatory requirements, including food contact approval associated regulations. In addition, there is a commercial and industrial need for rosin ester resins to be applied as a binder in high quality thermoplastic traffic line compounds that can permit higher filler loading in screed/extrusion applied thermoplastic formulations or which can serve as a binder for spray applied compounds and which can substantially increase the performance through improved adhesion to (mixed-in and drop-on) glass beads, non-skid aggregates and the road. In addition, there is a further need for rosin ester resins that may be utilized as additives for tires, e.g. as tread enhancement additives or as tackifying additives, which provide improved performance properties. In the tire industry, a tackifier such as a high softening point rosin ester can be useful during the tire forming process wherein parts such as the tread and side wall of a tire are attached together by the tackifier. There is a need to replace petroleum-based tackifiers in tires and other rubber compositions by environmental friendly resins like rosin esters as is for example described in: Physical Chemistry of Macromolecules: Macro to Nanoscales: Eds.: C. H. Chan, C. H. Chia, S. Thomas. Apple Academic Press/CRC Press, Taylor & Francis group, 2014, Chapter 17, p. 476-502, which is incorporated herein by reference in its entirety